Identify the song's copyright owner, which may or may not be the original recording artist. Consult the copyright notice at the bottom of the sheet of music. If the music appears in a collection, contact the publisher of the collection to identify the copyright owner.
Write to the copyright owner to request permission to arrange the musical piece. Identify yourself and the name of the composition that you wish to arrange. Indicate that the arranged music will be recorded for sale and specify the number of copies to be recorded. Depending on the copyright owner, you may be required to pay a fee for permission. Obtain a written authorization from the copyright owner as proof that you are authorized to arrange the work.
Reproduce permissions and include with each CD or recording of the the musical arrangement that you sell. For instance, indicate on the jacket or CD cover that permission was granted by the copyright owners to arrange works. Identify copyright owners by name, rather than simply stating that permission was obtained.
Advertise your musical arrangement as such. Avoid misrepresenting the music as a performance of the original piece.